From the root ب خ ر (b ḵ r), related to vapor. Compare with بَخَرَ (baḵara, “to boil”), تَبَخَّر (tabaḵḵar, “to evaporate, to perfume with incense”) and بُخُور (buḵūr, “incense”).
Practice speaking and memorizing "بخار" and many other words and sentences in Arabic.